Illumination apparatus confining light by total internal reflection and methods of forming the same

ABSTRACT

In various embodiments, an illumination apparatus includes an air gap between a sub-assembly and a waveguide attached thereto at a plurality of discrete attachment points, as well as a bare-die light-emitting diode encapsulated by the waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 61/560,293, filed on Nov. 16, 2011, the entiredisclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

In various embodiments, the present invention relates to artificialillumination, and in particular to an illumination apparatus confininglight therewithin by total internal reflection.

BACKGROUND

Light-emitting diodes (LEDs) are gradually replacing incandescent lightbulbs in various applications, including traffic signal lamps,large-sized full-color outdoor displays, various lamps for automobiles,solid-state lighting devices, flat panel displays, and the like.Conventional LEDs typically include a light-emitting semiconductormaterial, also known as the bare die, and numerous additional componentsdesigned for improving the performance of the LED. These components mayinclude a light-reflecting cup mounted below the bare die, a transparentencapsulation (typically silicone) surrounding and protecting the baredie and the light reflecting cup, and electrical leads for supplying theelectrical current to the bare die. The bare die and the additionalcomponents are efficiently packed in an LED package.

LEDs also represent an attractive alternative light source for generallighting applications and for backlights for liquid crystal displays,where they enable extremely low-thickness (or “low-profile”) solutions.One conventional geometry for such illumination solutions is theso-called edge-lit configuration, in which a packaged LED is attached tothe shorter, narrow side (or “face”) of a waveguide, and the light isemitted through the broader “top” face of the waveguide. Increasedcoupling efficiencies may be obtained by embedding the bare LED diewithin the waveguide itself, rather than by separately encapsulating orpackaging the die before coupling it to the waveguide. However, sincethe geometric dimensions of a waveguide typically are far larger thanthose of the LED die, it is often challenging to achieve and maintainthe high coupling efficiency enabled by embedding the bare die whilealso forming a strong mechanical connection between the variouscomponents of the completed system. The LED die is typically mounted ona platform, or a “sub-assembly” that provides mechanical support andelectrical connectivity to an external power source. The presence andgeometry of the LED sub-assembly may present difficulties whenattempting to embed the LED die within the waveguide with high couplingefficiency.

FIGS. 1A (cross-section) and 1B (bottom view) depict an illuminationdevice 100 that features an LED 105 mounted on a sub-assembly 110 andcoupled within a recess 115 in a waveguide 120. As shown, the waveguide120 has the shape of a thin plate with flat top and bottom faces, whichmay be parallel, as shown, or may be angled toward each other, givingthe waveguide 120 the shape of a wedge. Mirrors (or mirror coatings) maybe present along the bottom and side faces of waveguide 120. Duringoperation of the illumination device 100, light from the LED 105 iscoupled in to an input region 130 of the waveguide 120 via an inputcoupling element 135. The light then propagates toward an output region140 by means of total internal reflection (TIR) off of the top andbottom faces of the waveguide 120. (As known to those of skill in theart, TIR depends at least on the refractive-index difference of twomaterials at the boundary therebetween, as well as the angle of thelight impinging upon the boundary.) In the output region 140, the lightis out-coupled from the waveguide 120 by, e.g., embedded scatteringelements 145 that disrupt the TIR propagation, resulting in emittedlight 150.

Devices such as illumination device 100 present an extremely difficultchallenge—the need to, within the small thickness of the waveguide 120,convert light not emitted from the LED 105 in the TIR condition intolight propagating in waveguide 120 via TIR. This conversion generallymust be performed within a very small area in order to preventadditional loss of light from impingements of light on the waveguidefaces in non-TIR conditions. This, in turn, constrains the area of theLED sub-assembly 110, as the top face of the sub-assembly 110 generallydoes not reflect light at TIR conditions and/or may even absorb lightfrom the LED 105, diminishing overall efficiency.

However, for devices such as illumination device 100 to have adequatemechanical stability, the area of the “joint” between the waveguide 120and the LED sub-assembly 110 is typically much larger than that of theLED die itself, resulting in the above-described efficiency-diminishingarea of the sub-assembly surrounding the LED die. This additionalsub-assembly area increases the cross-section of contact between thewaveguide and the sub-assembly, strengthening the connection, but alsoresults in decreased input coupling efficiency.

FIG. 2 depicts one conventional approach to addressing this trade-off,in which only the “optical connection” (i.e., the proximity enablingin-coupling of light from the LED) between the LED and the waveguide ismade in immediate proximity to the LED, and the mechanical supportbetween the waveguide and the sub-assembly is provided separately. Asshown, an illumination device 200 features an LED 210 mounted on alarger sub-assembly 220 that is joined to a waveguide 230. The LED 210is flanked by two mechanical connections 240 that provide mechanicalsupport when the sub-assembly 220 is joined to the waveguide 230. Inmany such designs, it is recognized that light emitted from the LED inone or more lateral directions (e.g., the indicated x-direction) willnot reach the output region even in the absence of mechanicalconnections that may block or absorb such light. Specifically, much ofthe light reaching the side faces of the waveguide in such directionswill not reach the output region due to their multiple reflections, inthe x-direction, in the input-region. Thus, conventional designs mayplace the mechanical connections 240 in such locations, as light lostvia interaction therewith may well not have been efficiently coupledinto the bulk of the waveguide anyway; thus, losses associated with themechanical connections may have little additional impact on the inputcoupling efficiency.

Exacerbating the impact of mechanical supports on the input couplingefficiency is the fact that the optical connection between the LED andthe waveguide is typically achieved via a “dam and fill” process, inwhich a low-viscosity dam of encapsulant is formed between (and incontact with) the sub-assembly and the waveguide and then filled withhigher-viscosity index-matching material. Such processes fill the entirein-coupling region near the LED with the index-matching material, whichincreases the size of the surrounding region incapable of TIR-basedconfinement of the LED light (because, since the index-matching materialcontacts the waveguide and the light-absorbing sub-assembly, lightpropagating toward the sub-assembly is simply absorbed or otherwise lostrather than reflected into a TIR mode and efficiently in-coupled). Thus,in view of the challenges and disadvantages of conventionalwaveguide-based illumination devices described above, there is a needfor illumination devices having increased mechanical stability withoutassociated in-coupling losses that adversely impact overall efficiency.

SUMMARY

In accordance with various embodiments of the present invention,illumination devices achieve sufficient mechanical stability while alsominimizing the contact area between the waveguide and the LEDsub-assembly. Rather than utilize a dam-and-fill encapsulation scheme,embodiments of the invention encapsulate the bare LED die either in thewaveguide material itself (i.e., with no gaps or other materialstherebetween) or in one or more index-matching materials between the LEDdie and the waveguide material. In the latter case, for example, the LEDdie may be encapsulated in index-matching material on the LEDsub-assembly and then immerse the encapsulated LED die in additionalindex-matching material (which may be the same as or different from thefirst index-matching material) present in a recess (and in someembodiments, present only in the recess) in the waveguide. Thisstructure enables the formation of an air gap between the waveguide andthe LED sub-assembly during attachment thereof, thereby enabling TIR oflight in the waveguide in the vicinity of the sub-assembly. That is,light is confined due to the refractive-index difference between thewaveguide and the air gap, rather than being absorbed or otherwise lostvia propagation directly into or scattering from a sub-assembly inintimate contact with the waveguide. Furthermore, the index-matchingmaterial within the recess may not contact the sub-assembly afterimmersion of the LED die, thereby maintaining an air gap therebetweenacross as much area as possible. Thus, the minimized contact areabetween the LED die and the index-matching material reduces or evensubstantially eliminates in-coupling losses associated with lightpropagation to the sub-assembly.

Preferably, the recess in the waveguide has a cross-sectional areaand/or volume that is minimized while still accommodating immersion ofthe LED die therein in order to minimize any “non-TIR region,” i.e., aregion within the illumination device where in-coupled like is notconfined by TIR. The index-matching materials preferably have refractiveindices between those of the LED die itself and the waveguide in orderto optimize extraction of light from the LED die and coupling of thelight into the waveguide. Once the LED die is immersed in theindex-matching material in the recess, the surface of the index-matchingmaterial surrounding the LED die is preferably substantially parallel tothe bottom face of the waveguide, and thereby itself forms a TIRconfinement region (also due to the air gap therebelow). Thus, not onlydoes the bottom surface of the waveguide around the recess confine lightby TIR, but a portion of the recess itself does as well via theindex-matching material.

Preferably, bare LED dies utilized in embodiments of the presentinvention have rectangular cross-sections with the longer face parallelto the x direction (see FIG. 2), i.e., facing toward the output regionof the waveguide. Thus, such LED dies typically emit more light in adesired direction toward the output region.

In an aspect, embodiments of the invention feature an illuminationdevice including or consisting essentially of a waveguide, asub-assembly attached to a first surface of the waveguide only at aplurality of discrete attachment points, and a bare-die LED mechanicallycoupled to the sub-assembly and disposed within the waveguide, lightemitted by the bare-die LED being in-coupled into the waveguide. An airgap separates the first surface of the waveguide and the sub-assemblybetween the plurality of discrete attachment points.

Embodiments of the invention include one or more of the following in anyof a variety of combinations. A spacer may mechanically couple thebare-die LED to the sub-assembly and raise the bare-die LED above thetop surface of the sub-assembly. The waveguide may include a secondsurface for emitting light emitted by the bare-die LED and propagatingwithin the waveguide. The second surface may be opposite the firstsurface, and the first and second surfaces may even be substantiallyplanar. Alternatively, the first and second surfaces may be adjoiningand not parallel, e.g., substantially perpendicular to each other. Anin-coupling element (e.g., a reflector, a prism, and/or one or morescattering elements) for converting light emitted by the bare-die LEDfrom an unconfined mode to a confined mode may be disposed in thewaveguide above the bare-die LED. The waveguide may have a secondsurface opposite the first surface, and a substantially opaque absorberfor blocking propagation of light through a second surface of thewaveguide may be disposed over the second surface. The absorber may beattached to the second surface only at a second plurality of discreteattachment points; an air gap may thus be disposed between the secondsurface of the waveguide and the absorber between the second pluralityof discrete attachment points.

An adhesive material (e.g., a flexible adhesive) may be disposed at eachof the plurality of discrete attachment points. The waveguide may definea plurality of protrusions from the first surface; each of theprotrusions may be disposed at one of the discrete attachment points,and the maximum height of the protrusions above the first surface may beapproximately equal to the thickness of the air gap. A plurality ofdiscrete separators may be disposed between the first surface and thesub-assembly; each of the separators may be disposed at one of thediscrete attachment points, and the thickness of the separators may beapproximately equal to a thickness of the air gap. Each separator may beattached to the first surface and/or to the sub-assembly with anadhesive material. At least one discrete attachment point may bedisposed on a relief defined by at least one relief trench extendingthrough the thickness of the sub-assembly. The relief may be elasticallydeformable in at least a first direction substantially perpendicular tothe first surface, and the relief may even be elastically deformable ina second direction substantially perpendicular to the first direction.At least one discrete attachment point may not be disposed on a reliefand may thereby not be elastically deformable. The plurality of discreteattachment points may be disposed in a line substantially perpendicularto a propagation direction extending from the bare-die LED to an outputregion of the waveguide (e.g., a portion of the second surface of thewaveguide spaced away from the bare-die LED and any recess in thewaveguide).

The waveguide may define a recess in the first surface, and the bare-dieLED may be disposed within the recess. At least one index-matchingmaterial may fill at least a portion of the recess and encapsulate thebare-die LED, thereby facilitating in-coupling of light emitted by thebare-die LED into the waveguide. A free surface of at least one of theindex-matching materials in the recess may be spaced away from thesub-assembly, thereby defining a second air gap between the sub-assemblyand the free surface. The free surface may be substantially parallel tothe first surface of the waveguide. In a propagation direction extendingfrom the bare-die LED to an output region of the waveguide, a dimensionof the recess may be no more than three times (or no more than twotimes, or even approximately equal to) a dimension of the bare-die LEDin the propagation direction.

In another aspect, embodiments of the invention feature a method offorming an illumination device incorporating a bare-die LED mechanicallycoupled to a sub-assembly. The sub-assembly is attached to a firstsurface of a waveguide at only a plurality of discrete attachmentpoints, thereby defining an air gap disposed between the first surfaceof the waveguide and the sub-assembly between the plurality of discreteattachment points. The bare-die LED is encapsulated within a recessdefined by the waveguide in the first surface, thereby facilitatingin-coupling of light emitted by the bare-die LED into the waveguide.

Embodiments of the invention include one or more of the following in anyof a variety of combinations. Encapsulating the bare-die LED may includeor consist essentially of at least partially surrounding the bare-dieLED with at least one index-matching material. At least one suchindex-matching material may be disposed within the recess before thesub-assembly is attached to the first surface of the waveguide.Encapsulating the bare-die LED may include or consist essentially of atleast partially surrounding the bare-die LED with a first index-matchingmaterial prior to attachment of the sub-assembly to the first surface ofthe waveguide, disposing a second index-matching material within therecess prior to attachment of the sub-assembly to the first surface ofthe waveguide, and at least partially surrounding the bare-die LED and aportion of the first index-matching material with the secondindex-matching material during attachment of the sub-assembly to thefirst surface of the waveguide. At least one discrete attachment pointmay be disposed on a relief defined by at least one relief trenchextending through the thickness of the sub-assembly. The relief may beelastically deformable in at least a first direction substantiallyperpendicular to the first surface, and the relief may even beelastically deformable in a second direction substantially perpendicularto the first direction. At least one discrete attachment point may notbe disposed on a relief and may thereby not be elastically deformable.Attaching the sub-assembly to the first surface of the waveguide mayinclude or consist essentially of forming the plurality of discreteattachment points via dispersal of an adhesive between the sub-assemblyand the waveguide at each of the discrete attachment points.

In yet another aspect, embodiments of the invention include a method offorming an illumination device incorporating a bare-die LED. Thebare-die LED is encapsulated within a waveguide by disposing thebare-die LED within a waveguide material such that the waveguidematerial directly contacts, without a gap therebetween, either thebare-die LED or an index-matching material disposed around and in directcontact with the bare-die LED.

Embodiments of the invention include one or more of the following in anyof a variety of combinations. A sub-assembly may be attached to a firstsurface of the waveguide at only a plurality of discrete attachmentpoints, thereby defining an air gap disposed between the first surfaceof the waveguide and the sub-assembly between the plurality of discreteattachment points. The waveguide may define a plurality of protrusionsfrom the first surface, each protrusion being disposed at one of thediscrete attachment points; the maximum height of the protrusions abovethe first surface may be approximately equal to the thickness of the airgap. The waveguide and the sub-assembly may be brought into contact atthe plurality of discrete attachment points substantially simultaneouslywith the bare-die LED being encapsulated within the waveguide. At leastone discrete attachment point may be disposed on a relief defined by atleast one relief trench extending through the thickness of thesub-assembly. The relief may be elastically deformable in at least afirst direction substantially perpendicular to the first surface, andthe relief may even be elastically deformable in a second directionsubstantially perpendicular to the first direction. At least onediscrete attachment point may not be disposed on a relief and maythereby not be elastically deformable. The bare-die LED may beencapsulated substantially simultaneously with formation of thewaveguide. The index-matching material may be disposed around thebare-die LED prior to encapsulating the bare-die LED within thewaveguide.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “substantially” and “approximately” mean±10%, and in someembodiments, ±5%, unless otherwise indicated. The term “consistsessentially of” means excluding other materials or structures thatcontribute to function, unless otherwise defined herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIGS. 1A and 1B are a cross-section and a bottom view, respectively, ofan LED-based illumination device;

FIG. 2 is a plan view of an LED-based illumination device;

FIGS. 3-5 are cross-sectional views of illumination devices having oneor more LEDs optically coupled within a waveguide in accordance withvarious embodiments of the present invention;

FIG. 6 is a perspective view of an illumination device in accordancewith various embodiments of the present invention; and

FIGS. 7-9 are perspective views of illumination devices incorporatingfeatures to reduce thermally induced stresses in accordance with variousembodiments of the present invention.

DETAILED DESCRIPTION

FIG. 3 depicts an illumination device 300 in accordance with embodimentsof the present invention. As shown, illumination device 300 includes oneor more bare LED dies 305 mounted on a spacer 310 on an LED sub-assembly315. The spacer (which may be different and discrete from thesub-assembly 315, or may simply be a raised portion thereof) elevatesthe LED die 305 above the plane of the sub-assembly 315, which ispreferably otherwise substantially planar, thus facilitating the opticalconnection between the LED 305 and a waveguide 320. The waveguide 320 ispreferably substantially optically transparent, but may also incorporatevarious features (e.g., scatterers, reflectors, etc.) for thein-coupling, reflection, and out-coupling of light confined therein. Thewaveguide 320 may include or consist essentially of one or morepolymeric materials, e.g., latex, polyvinylchloride, nitrile,chloroprene (Neoprene), poly(cis-isoprene), poly(2,3-dimethylbutadiene),poly(dimethylsiloxane), ethylene/vinyl acetate copolymer-40% vinylacetate, ethylene/vinyl acetate copolymer-30% vinyl acetate,poly(butadiene-co-acrylonitrile), natural rubber, poly(chloroprene),polymethylmethacrylate, and/or polycarbonate. The sub-assembly 315 mayinclude or consist essentially of one or more suitably rigid materials,e.g., a printed circuit board (PCB) or a metal-core PCB.

In various embodiments of the present invention, the LED die 305 isencapsulated with an index-matching material 325 prior to the connectionof sub-assembly 315 with waveguide 320. The index-matching material 325preferably has an index of refraction between those of the LED die 305and the waveguide 320 in order to facilitate light extraction from theLED die 305 and in-coupling of the light into the waveguide 320. Theindex-matching material 325 may also advantageously cover any wires,other electrical connections, contact pads, and the like, and may thusprotect such elements from damage and/or elemental exposure. As shown,the index-matching material 325 may additionally coat at least a portion(or even substantially all of) the spacer 310, and may even coatportions of the sub-assembly 315 surrounding the spacer 310.Index-matching material 325 may be dispensed over the LED die 305 inliquid or gel form, and then partially or fully cured beforesub-assembly 315 is connected to the waveguide 320.

In various embodiments of the invention, once the LED die 305 isencapsulated with the index-matching material 325, LED die 305 ispositioned into a recess (or “cavity”) 330 in the waveguide 320, and ahigh-coupling-efficiency optical connection between the LED die 305 andthe waveguide is enabled via an index-matching material 335. Theindex-matching material 335 may include or consist essentially of thesame material as index-matching material 325, or it may be a differentmaterial. For example, the index of refraction of index-matchingmaterial 335 may be between those of index-matching material 325 andwaveguide 320. Index-matching materials 325, 335 may each (or both)include or consist essentially of, e.g., silicone and/or epoxy.

In one embodiment of the invention, the index-matching material 335 isdispensed into the recess 330 prior to the LED die 305 being positionedtherein. The waveguide 320 may be positioned with the recess 330 openingupwards, and may be positioned such that the bottom surface of thewaveguide 320 is substantially perpendicular to the force of gravity.Thus, once the LED die 305 is positioned within the recess 330, theindex-matching material 335 settles such that the surface thereof issubstantially parallel to the bottom surface of the waveguide 320 (i.e.,the surface of the waveguide 320 in which the recess 330 is formed). Thesub-assembly 310 is then positioned proximate the waveguide 320 suchthat the LED die 305 is positioned within the recess 330 and surrounded(at least light-emitting portions thereof) by the index-matchingmaterial 335. The index-matching material 335 may initially be dispensedin a liquid or gel form and may be fully or partially cured once LED die305 is positioned therewithin. As mentioned, after the opticalconnection of LED die 305 into the waveguide 320 (via the index-matchingmaterial 335), the exposed surface of the index-matching material 335 ispreferably substantially parallel to the bottom surface of the waveguide320 and spaced away from the sub-assembly 315. As shown in FIG. 3, theexposed surface of the index-matching material 335 is not necessarilycoplanar with the bottom surface of the waveguide 320 (thussubstantially filling the recess 330), but in some embodiments it iscoplanar therewith.

In an alternative embodiment of the invention, the LED die 305 ispositioned within a substantially empty recess 330 by bringing thesub-assembly 315 proximate the waveguide 320. Then, the index-matchingmaterial 335 may be injected into the partially occupied recess 330,surrounding the LED die 305. As described above, the index-matchingmaterial 335 may be dispensed as a liquid or gel and partially or fullycured afterwards. Also, after the index-matching material 335 isinjected around the LED die 305, the exposed surface of theindex-matching material is preferably substantially parallel to thebottom surface of the waveguide 320 and spaced away from thesub-assembly 315, as described above.

In some embodiments of the invention, as shown in FIG. 3, the LED die305 is positioned within the recess 330 substantially spaced away fromthe walls and the surface of the waveguide 320 disposed above the LEDdie 305 and defined by recess 330. In other embodiments the top surfaceof LED die 305 (or the index-matching material 325, if present on thetop surface of LED die 305) is in direct contact with the surface ofwaveguide 320 within the recess 330; thus, light emitted from the topsurface of the LED die 305 is coupled directly into the waveguide 320without necessarily traversing index-matching material. In either typeof embodiment there is preferably no empty gap between a light-emittingportion of the LED die 305 and the waveguide 320—such space is eithernonexistent or filled with one or more index-matching materials.

Preferably, TIR within the waveguide 320 in the proximity of LED die 305and sub-assembly 315 is facilitated by the attachment of waveguide 320to sub-assembly 315 such than an air gap 340 is formed and maintainedtherebetween. TIR in this region of the waveguide 320 enables theconfinement of light within the waveguide 320, thereby increasing theinput coupling efficiency of illumination device 300 (i.e., the amountof light emitted by LED die 305 that is successfully confined within thewaveguide 320). The air gap 340 may have a thickness in the range of,e.g., approximately 1 μm to approximately 1000 μm. In order to attachthe sub-assembly 315 to the waveguide 320 with sufficient mechanicalstability but while maintaining the air gap 340, one or more regions ofan adhesive 345 may be disposed between the two parts. The adhesive 345is preferably spaced away from the recess 330 and/or shaped to have afairly small area of contact with the waveguide 320, thereby minimizingany absorptive and/or scattering losses resulting from interaction ofthe light in the waveguide 320 with the adhesive 345. The adhesive 345may have an index of refraction smaller than that of the waveguide 320in order to help facilitate TIR within the waveguide 320 in the vicinityof adhesive 345. The adhesive 345 may be disposed in one or more regionssubstantially perpendicular to the intended light path from the LED die305 to the emission surface of the waveguide 320, e.g., as shown for thereinforcements in FIG. 2.

Illumination device 300 may also incorporate one or more in-couplingelements 350 disposed above the recess 330 (and preferably proximate oreven at the surface of waveguide 320 opposite recess 330). Thein-coupling element 350 may include or consist essentially of, e.g., areflector (which may be planar and/or curved), a prism, or one or morescattering elements such as bubbles or surface features such ashemispheres. In-coupling element 350 facilitates the in-coupling oflight emitted by the LED die 305 into the bulk of the waveguide 320 byredirecting one or more portions of light not already propagatingtherewithin in a TIR condition. Because the in-coupling element 350(and/or other portions of the waveguide 320) may be an imperfectreflector and/or may allow some finite amount of light to propagatetherethrough, the illumination device 300 may also incorporate anabsorber 355 that absorbs and/or reflects such light, thereby preventingits escape into the surrounding ambient. The absorber is preferablysubstantially opaque and may include or consist essentially of, e.g.,one or more metallic, plastic, and/or ceramic materials. As with theattachment of the sub-assembly 315 to waveguide 320, the absorber 355may be attached via, e.g., one or more regions of an adhesive 360 (whichmay be the same as or different from adhesive 345) such that an air gap365 is formed and maintained between waveguide 320 and absorber 355. Theair gap 365 may have a thickness in the range of, e.g., approximately 1μm to approximately 1000 μm, and it facilitates the TIR confinement oflight within the waveguide 320. A center portion of absorber 355disposed directly over the recess 330 and/or LED die 305 may besubstantially reflective to reflect light back into waveguide 320. Asdescribed above for adhesive 345, the adhesive 360 may be disposed inone or more regions substantially perpendicular to the intended lightpath from the LED die 305 to the emission surface of the waveguide 320,e.g., as shown for the reinforcements in FIG. 2.

In some embodiments of the invention the waveguide 320 has a coefficientof thermal expansion (CTE) different from that of the sub-assembly 315and/or the absorber 355. In such embodiments, the index-matchingmaterials 325, 335 and/or the adhesives 345, 360 may be selected tomitigate at least a portion of the CTE mismatch between the variouscomponents of illumination system 300. For example, either or both ofadhesives 345, 360 may be substantially flexible, and any of adhesives345, 360 and/or index-matching materials 325, 335 may be at leastpartially gelatinous to thereby absorb CTE-mismatch stresses and preventthe debonding of waveguide 320 from sub-assembly 315 and/or absorber355. Embodiments of the invention may incorporate other features toreduce CTE-mismatch stresses, as described below with reference to FIGS.6-9.

As shown in FIG. 4, embodiments of the invention feature LED dies 305“edge-coupled” into waveguide 320, i.e., optically connected to a faceof waveguide 320 that adjoins (i.e., is not opposite) the top,light-emitting surface thereof. In FIG. 4, an illumination device 400features an LED die 305 optically connected to the waveguide 320 viaindex-matching material 335 disposed within an edge recess 400,substantially as described above for recess 330.

FIG. 5 depicts an exemplary illumination device 500 in which theadhesive 345 is replaced with or supplemented by one or more protrusions510 that are intimate portions of and protrude from the body ofwaveguide 320, thereby facilitating formation and maintenance of the airgap 340 when waveguide 320 is attached to the sub-assembly 315. In otherembodiments of the invention, the protrusions 510 are portions of andprotrude from the sub-assembly 315 (in addition to or instead ofprotruding from waveguide 320). As shown for the reinforcements in FIG.2, the protrusions 510 may be disposed in one or more regionssubstantially perpendicular to the intended light path from the LED die305 to the emission surface of the waveguide 320. In some embodiments,the protrusions 510 are supplemented or replaced by discrete separatorsat the attachment points. For example, both the waveguide 320 and thesub-assembly 315 may be substantially planar in the vicinity of theattachment points, and the separators may be attached to both thewaveguide 320 and sub-assembly 315 at those points by, e.g., anadhesive. In such cases the thickness of the separators generallydefines the thickness of the air gap 340. In other embodiments, either(or both) of the sub-assembly 315 or waveguide 320 may incorporateprotrusions 510 that have a thickness smaller than that of the desiredair gap 340, and discrete separators are utilized with (and, e.g.,adhered to) those protrusions to increase the size of the air gap 340.The separators may include or consist essentially of any suitably rigidmaterial, e.g., one or more of the materials of sub-assembly 315, spacer310, or waveguide 320.

As also shown in FIG. 5, the waveguide 320 may be formed directly aroundthe LED die 305 and/or the index-matching material 325 (if present);thus, the bare LED die 305 or the LED die encapsulated by index-matchingmaterial 325 is encapsulated by the waveguide material itself. Forexample, the waveguide 320 may be initially formed, e.g., by injectionmolding, such that it at least partially surrounds LED die 305, or theLED die 305 may be placed within the waveguide material while thewaveguide material is in a liquid or gelatinous state, whereupon thematerial is cured to form waveguide 320 encapsulating the LED die 305.In such embodiments, the bare LED die 305 may have disposed thereupon alayer of index-matching material 325 (as depicted in FIG. 5) or may bedisposed in direct contact (i.e., with no intervening gaps or othermaterials) with the waveguide 320, which may then not define therewithinany other sort of recess.

Embodiments of the present invention may also incorporate variousfeatures to minimize the impact of CTE mismatch between variouscomponents. As shown in FIG. 6, an exemplary illumination device 600 mayinclude or consist essentially of a sub-assembly 315 attached to awaveguide 320 via two or more joining elements 610. (Features such asLED die 305 and recess 330 are not shown in FIGS. 6-9 for clarity butmay be present as detailed above.) The joining elements 610 may includeor consist essentially of, e.g., adhesive 345 and/or protrusions 510 asdescribed above. The sub-assembly 315 and the waveguide 320 typicallyhave different CTEs, and there is thus a finite amount of CTE mismatchtherebetween that may result in deleterious stresses or even debondingduring or after thermal cycles experienced by illumination device 600.As described above regarding FIGS. 3 and 4, the sub-assembly 315 and thewaveguide 320 are typically in contact only at the joining elements 610and there is otherwise an air gap between sub-assembly 315 and waveguide320.

The CTE mismatch between the sub-assembly 315 and the waveguide 320 mayarise due to these elements comprising or consisting of differentmaterials, as mentioned above. For example, the sub-assembly 315 mayinclude or consist essentially of a PCB or a material (such as a ceramicmaterial) utilized in the formation of PCBs, and may thus have a CTE inthe range of approximately 15 to 17 ppm/° C. Sub-assembly 315 may eveninclude a metal plate, and may include or consist essentially of ametal-core PCB, where the metal may be, e.g., aluminum (having a CTE ofapproximately 24 ppm/° C.) and/or copper (having a CTE of approximately18 ppm/° C.). In contrast, the waveguide 320 may include or consistessentially of, e.g., polymethylmethacrylate (PMMA), which has a CTE ofapproximately 70 ppm/° C.).

In a non-limiting exemplary embodiment presented to demonstratepotential CTE mismatch-related issues, the CTE-related stressexperienced in an illumination device 600 (generally at the two or morejoining elements 610) may be calculated. In the exemplary embodiment,the sub-assembly 315 has a CTE_(SA) of approximately 24 ppm/° C. and thewaveguide 320 has a CTE_(W) of approximately 70 ppm/° C. For example, ifa distance L between two joining elements 610 is 24 mm, then for atemperature change ΔT of 80° C., the difference in linear expansion ΔLbetween the two elements corresponds to:

ΔL=(CTE_(W)−CTW_(SA))×ΔT×L=88 μm.

The resulting stress a in the waveguide 320, which has a Young's modulusE_(W) of, e.g., 3.2 Gigapascals (GP), corresponds to:

σ=(ΔL/L)×E _(W)=11.7 Megapascals (MP).

Assuming that the two joining elements 610 in question have surfaceareas S of approximately 6 mm², then the force experienced at thejoining elements 610 (and also the waveguide 320) corresponds toF=σ×S=approximately 70 Newtons (N).

As shown in FIG. 7, embodiments of the invention advantageouslyincorporate features to mitigate CTE-mismatch stresses. In accordancewith various embodiments, an illumination device 700 includessub-assembly 315 and waveguide 320, similar to illumination device 600described above. As in illumination device 600, the sub-assembly 315 andwaveguide 320 are attached at two or more joining elements 610,indicated in FIG. 7 as joining elements 610-1 and 610-2. As shown,illumination device 700 features one or more release trenches 710 (thatpreferably extend through the thickness of sub-assembly 315), therebyforming reliefs 720 on which one or more joining elements 610-2 aredisposed The reliefs 720 remain connected to the bulk of thesub-assembly 315 along one dimension and at at least one point, but aresubstantially free to move along other dimensions (i.e., dimensionsperpendicular to the direction of connection) and thus providecompliancy to the connection between sub-assembly 315 and waveguide 320.The reliefs 720 may be free to move elastically and may therefore beconsidered to be elastic springs that act to relieve CTE-mismatch stressduring thermal cycles.

In order to maintain the alignment between sub-assembly 315 andwaveguide 320, at least one joining element 610 (indicated as joiningelement 610-1) is maintained as a substantially rigid connection, i.e.,a relief 720 is not formed thereunder. The beneficial impact of thereliefs 720 may be calculated utilizing many of the same exemplaryvalues utilized above in relation to FIG. 6. For a relief having alength d of 10 mm a width w of 1.5 mm, and a thickness b of 1.3 mm, forthe above-calculated displacement between joining elements 610 of 88 μm,the force induced at the end of the relief (and thus experienced at thejoining element 610) is only approximately 5 N, a much smaller valuethan the 70 N experienced in the absence of the relief trenches 710.

As shown in FIG. 7, and as also shown in FIGS. 8 and 9, the alignmentand sizes of the relief trenches 710 may vary and may thus form avariety of differently sized and shaped reliefs 720 upon which compliantjoining elements 610-2 are formed. FIGS. 8 and 9 also schematicallydepict an in-coupling region 800 and an out-coupling region 810 of thewaveguide 320. Typically light emitted by an LED die 305 is in-coupledinto the waveguide 320 in the in-coupling region 800, as shown in moredetail and described in reference to FIGS. 3-5, and light is not emittedfrom the waveguide 320 in in-coupling region 800. At least a portion (oreven all) of the in-coupling region 800 may be covered by an absorber355, as shown in FIG. 3. The in-coupled light propagates within thewaveguide 320 and is subsequently out-coupled (i.e., emitted) from,e.g., the top surface of the waveguide 320 in the out-coupling region810.

Illumination apparatuses in accordance with embodiments of the presentinvention may also incorporate one or more phosphors or otherphotoluminescent materials as described in U.S. patent application Ser.No. 13/255,113, filed Sep. 7, 2011, the entire disclosure of which isincorporated by reference herein.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

What is claimed is:
 1. An illumination device comprising: a waveguide; asub-assembly attached to a first surface of the waveguide only at aplurality of discrete attachment points, an air gap separating the firstsurface of the waveguide and the sub-assembly between the plurality ofdiscrete attachment points; and a bare-die light-emitting diode (LED)mechanically coupled to the sub-assembly and disposed within thewaveguide, light emitted by the bare-die LED being in-coupled into thewaveguide.
 2. The illumination device of claim 1, further comprising aspacer mechanically coupling the bare-die LED to the sub-assembly andraising the bare-die LED above a top surface of the sub-assembly.
 3. Theillumination device of claim 1, wherein the waveguide comprises,opposite the first surface, a second surface for emitting light emittedby the bare-die LED and propagating within the waveguide.
 4. Theillumination device of claim 3, wherein the first and second surfacesare substantially planar.
 5. The illumination device of claim 1, whereinthe waveguide comprises, adjoining and not parallel to the firstsurface, a second surface for emitting light emitted by the bare-die LEDand propagating within the waveguide.
 6. The illumination device ofclaim 5, wherein the first and second surfaces are substantiallyperpendicular to each other.
 7. The illumination device of claim 1,further comprising, disposed in the waveguide above the bare-die LED, anin-coupling element for converting light emitted by the bare-die LEDfrom an unconfined mode to a confined mode.
 8. The illumination deviceof claim 7, wherein the in-coupling element comprises at least one of areflector, a prism, or one or more scattering elements.
 9. Theillumination device of claim 1, further comprising, disposed over asecond surface of the waveguide opposite the first surface, asubstantially opaque absorber for blocking the propagation of lighttherethrough.
 10. The illumination device of claim 9, wherein theabsorber is attached to the second surface of the waveguide only at asecond plurality of discrete attachment points, an air gap beingdisposed between the second surface of the waveguide and the absorberbetween the second plurality of discrete attachment points.
 11. Theillumination device of claim 1, further comprising an adhesive materialdisposed at each of the plurality of discrete attachment points.
 12. Theillumination device of claim 11, wherein the adhesive is flexible. 13.The illumination device of claim 1, wherein the waveguide defines aplurality of protrusions from the first surface, each of the protrusionsbeing disposed at one of the discrete attachment points, a maximumheight of the protrusions above the first surface being approximatelyequal to a thickness of the air gap.
 14. The illumination device ofclaim 1, further comprising a plurality of discrete separators disposedbetween the first surface and the sub-assembly, each of the separatorsbeing disposed at one of the discrete attachment points, a thickness ofthe separators being approximately equal to a thickness of the air gap.15. The illumination device of claim 14, wherein each separator isattached to the first surface and to the sub-assembly with an adhesivematerial.
 16. The illumination device of claim 1, wherein at least onediscrete attachment point is disposed on a relief defined by at leastone relief trench extending through a thickness of the sub-assembly, therelief being elastically deformable in at least a first directionsubstantially perpendicular to the first surface.
 17. The illuminationdevice of claim 16, wherein the relief is elastically deformable in asecond direction substantially perpendicular to the first direction. 18.The illumination device of claim 16, wherein at least one discreteattachment point is not disposed on a relief and thereby is notelastically deformable.
 19. The illumination device of claim 1, whereinthe plurality of discrete attachment points are disposed in a linesubstantially perpendicular to a propagation direction extending fromthe bare-die LED to an output region of the waveguide.
 20. Theillumination device of claim 1, wherein the waveguide defines a recessin the first surface, and the bare-die LED is disposed within therecess, further comprising at least one index-matching material fillingat least a portion of the recess and encapsulating the bare-die LED,thereby facilitating in-coupling of light emitted by the bare-die LEDinto the waveguide.
 21. The illumination device of claim 20, wherein afree surface of at least one said index-matching material in the recessis spaced away from the sub-assembly, thereby defining a second air gapbetween the sub-assembly and the free surface.
 22. The illuminationdevice of claim 21, wherein the free surface of the at least oneindex-matching material is substantially parallel to the first surfaceof the waveguide.
 23. The illumination device of claim 20, wherein, in apropagation direction extending from the bare-die LED to an outputregion of the waveguide, a dimension of the recess is no more than threetimes a dimension of the bare-die LED in the propagation direction. 24.The illumination device of claim 23, wherein the dimension of the recessin the propagation direction is no more than two times the dimension ofthe bare-die LED in the propagation direction.
 25. The illuminationdevice of claim 24, wherein the dimension of the recess in thepropagation direction is approximately equal to the dimension of thebare-die LED in the propagation direction.
 26. A method of forming anillumination device incorporating a bare-die light-emitting diode (LED)mechanically coupled to a sub-assembly, the method comprising: attachingthe sub-assembly to a first surface of a waveguide at only a pluralityof discrete attachment points, thereby defining an air gap disposedbetween the first surface of the waveguide and the sub-assembly betweenthe plurality of discrete attachment points; and encapsulating thebare-die LED within a recess defined by the waveguide in the firstsurface, thereby facilitating in-coupling of light emitted by thebare-die LED into the waveguide.
 27. The method of claim 26, whereinencapsulating the bare-die LED comprises at least partially surroundingthe bare-die LED with at least one index-matching material.
 28. Themethod of claim 27, wherein at least one said index-matching material isdisposed within the recess before the sub-assembly is attached to thefirst surface of the waveguide.
 29. The method of claim 26, whereinencapsulating the bare-die LED comprises: at least partially surroundingthe bare-die LED with a first index-matching material prior toattachment of the sub-assembly to the first surface of the waveguide;disposing a second index-matching material within the recess prior toattachment of the sub-assembly to the first surface of the waveguide;and at least partially surrounding the bare-die LED and a portion of thefirst index-matching material with the second index-matching materialduring attachment of the sub-assembly to the first surface of thewaveguide.
 30. The method of claim 26, wherein at least one discreteattachment point is disposed on a relief defined by at least one relieftrench extending through a thickness of the sub-assembly, the reliefbeing elastically deformable in at least a first direction substantiallyperpendicular to the first surface.
 31. The method of claim 30, whereinat least one discrete attachment point is not disposed on a relief andthereby is not elastically deformable.
 32. The method of claim 26,wherein attaching the sub-assembly to the first surface of the waveguidecomprises forming the plurality of discrete attachment points viadispersal of an adhesive between the sub-assembly and the waveguide ateach of the discrete attachment points.
 33. A method of forming anillumination device incorporating a bare-die light-emitting diode (LED),the method comprising encapsulating the bare-die LED within a waveguideby disposing the bare-die LED within a waveguide material such that thewaveguide material directly contacts, without a gap therebetween, thebare-die LED or an index-matching material disposed around and in directcontact with the bare-die LED.
 34. The method of claim 33, furthercomprising attaching a sub-assembly to a first surface of the waveguideat only a plurality of discrete attachment points, thereby defining anair gap disposed between the first surface of the waveguide and thesub-assembly between the plurality of discrete attachment points. 35.The method of claim 34, wherein the waveguide defines a plurality ofprotrusions from the first surface, each of the protrusions beingdisposed at one of the discrete attachment points, a maximum height ofthe protrusions above the first surface being approximately equal to athickness of the air gap.
 36. The method of claim 34, wherein,substantially simultaneously, (i) the waveguide and the sub-assembly arebrought into contact at the plurality of discrete attachment points, and(ii) the bare-die LED is encapsulated within the waveguide.
 37. Themethod of claim 33, wherein at least one discrete attachment point isdisposed on a relief defined by at least one relief trench extendingthrough a thickness of the sub-assembly, the relief being elasticallydeformable in at least a first direction substantially perpendicular tothe first surface.
 38. The method of claim 37, wherein at least onediscrete attachment point is not disposed on a relief and thereby is notelastically deformable.
 39. The method of claim 33, wherein the bare-dieLED is encapsulated substantially simultaneously with formation of thewaveguide.
 40. The method of claim 33, further comprising disposing theindex-matching material around the bare-die LED prior to encapsulatingthe bare-die LED within the waveguide.